The Australian government is in clear breach of its international obligations by allowing the Western Australian shark cull, a conservation group has claimed, ahead of global protests against the policy.

Humane Society International said advice it had received from the United Nations showed that Australia was breaching the Convention on the Conservation of Migratory Species of Wild Animals by allowing the cull.

HSI claimed that the convention’s executive secretary wrote to Greg Hunt, the environment minister, in December about the policy but that Hunt had waited until 31 January to respond. Hunt’s office has been contacted to verify this.

According to the conservation group, advice it received from Melanie Virtue, from the convention secretariat, states: “The convention prohibits the taking of such species except under ‘extraordinary circumstances’,” and that “the ‘extraordinary circumstances’ clause in paragraph 5d of Article III of the convention has not been invoked by a CMS party, to the best of my knowledge”.

HSI, which has lodged a formal complaint with the UN about the cull, said Hunt should work with the convention secretariat to consult international experts on the policy.

“The legal and scientific advice is clear, the ‘extraordinary circumstance’ provisions under both the Environmental Protection and Biodiversity Conservation Act, 1999, which Mr Hunt has already utilised, and CMS, cannot be applied to WA’s shark culling program,” said Michael Kennedy, HSI’s campaign director.

“The fact that it has taken Australia more than two months to write to the CMS secretariat is an indication of their failure to consider all the legal obligations relating to the WA shark cull.”

The UN convention is aimed at protecting migratory species such as the great white shark. The great white is also protected under Australian law, but Hunt allowed WA an exemption in order to undertake its shark cull.

In a sign of the international chagrin at the policy, protest rallies will be held in San Francisco on Saturday, in Spain and Argentina on Sunday and in London on 1 March. A third series of Australian protests will also take place on the weekend, while work has begun on a legal challenge to the policy.

The South African government has already raised concern over WA’s plan, which involves the deployment of 72 baited hooks 1km out from popular beaches in the state.

Tiger, bull and great white sharks more than 3m in length caught on the hooks will be shot by contractors on patrol boats. However, most of the animals caught so far have been undersized tiger sharks.

“Worldwide, people are standing up to what’s happening in WA, the eyes of the world are on this,” scuba diver and protest organiser Natalie Banks told Guardian Australia.

“This is bigger than WA. The great whites are not just WA’s great whites, they also migrate to South Africa. People globally are becoming aware of what’s going on here.”

But the fisherman at the frontline of the controversial kill policy says he has been left alone by activists who had threatened to sabotage the operation.

The south-west contractor, who has asked not to be named, says he has seen very little direct opposition to his work on the water and he has been largely ignored since the initial furore over his lucrative contract last month.

“We have had very little issues with activism and I am very grateful for that,” the fisherman said.

More than 30 sharks are thought to have been caught on drum lines in West Australian waters and the government has confirmed that some were being cannibalised by larger sharks.